This invention relates to herbicidal compositions, and more particularly relates to herbicidal compositions containing one or more N-phosphonomethylglycine salts and a surfactant.
N-Phosphonomethylglycine, known in the agricultural chemical art as glyphosate, is a highly effective and commercially important phytotoxicant useful in controlling the growth of germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants. N-phosphonomethylglycine and its salts are conveniently applied in an aqueous solution as a post-emergent phytotoxicant for the control of numerous plant species. N-Phosphonomethylglycine and its salts are characterized by a broad spectrum activity, i.e., they control growth of a wide variety of plants.
Commercial formulations of N-phosphonomethylglycine are usually aqueous solutions wherein the N-phosphonomethylglycine is present as a herbicidally acceptable salt, such as an alkali metal salt, the ammonium, alkylsulfonium, or alkylphosphonium salt or the salt of an amine having a molecular weight of less than about 300. The monoisopropylamine salt of N-phosphonomethylglycine is the most widely used salt in such aqueous compositions. In addition, such compositions usually contain a surfactant to enhance the effectiveness of the N-phosphonomethylglycine when it is applied to the foliage of various plants. The most widely used surfactant in commercial compositions is an ethoxylated fatty amine.
It is known to those skilled in the art that a particular surfactant used in an aqueous composition 5 with a herbicide can enhance the effectiveness of the herbicide, whereas other surfactants have very little, if any beneficial effect, and in fact, some may be antagonistic. Wyrill and Burnside, Weed Science, Vol. 25 (1977), p 275-287, examined dilute N-phosphonomethylglycine salt solutions containing different classes of surfactants, including polyoxyethylene octadecyl methyl ammonium chlorides containing on average 2 and 15 oxyethylene units (ETHOQUAD 18/12 and ETHOQUAD 18/25, respectively). They did not examine or suggest the use of shorter-chain alkyl methyl ammonium chlorides. Some classes of surfactant were more effective than others in enhancing the herbicidal effect of N-phosphonomethylglycine (used as a solution of the isopropylamine salt), and Wyrill and Burnside concluded that an effective surfactant is a critical component of any aqueous composition containing N-phosphonomethylglycine. For example, ETHOQUAD 18/12 was relatively ineffective in enhancing phytotoxicity of N-phosphonomethylglycine to hemp dogbane, whereas in a separate experiment ETHOQUAD 18/25 was one of the most effective surfactants tested. In the latter experiment, the surfactant was used at a concentration of 1% of the spray solution, a very high concentration by comparison with that delivered by commercial agricultural formulations of N-phosphonomethylglycine at typical application rates in typical spray volumes. The ratio of ETHOQUAD 18/25 to N-phosphonomethylglycine (expressed as acid equivalent) was 6.7:1 at the higher N-phosphonomethylglycine rate applied, and 26.7:1 at the lower rate applied. This compares with surfactant/ N-phosphonomethylglycine ratios typically in the range from 1:4 to 4:1, most commonly around 1:2, delivered by commercial agricultural formulations.
Although certain surfactants may enhance the biological activity of N-phosphonomethylglycine when used at relatively high concentrations in dilute N-phosphonomethylglycine salt solutions (e.g., a tank mix), many of such surfactants are ineffective at low surfactant/N-phosphonomethylglycine ratios. However, it is impossible to incorporate surfactants into highly concentrated aqueous compositions of N-phosphonomethylglycine except at such low ratios. Even then, many surfactants are difficult to coformulate with concentrated aqueous solutions of N-phosphonomethylglycine salts because of adverse effects on viscosity, clarity, high and low temperature stability and other physical characteristics that are desired by the end user. In addition, while N-phosphonomethylglycine and its salts are known to be of very low toxicity and environmentally acceptable, many surfactants are relatively toxic to aquatic life, and/or cause irritation when in contact with the eye.
U.S. Pat. No. 4,075,002 to Drewe et al. and U.S. Pat. No. 4,525,200 to Kimpara et al. each disclose polyoxyethylene alkyl methyl ammonium chlorides in combination with other surfactants in aqueous herbicidal compositions. However, neither of these references disclose the use of polyoxyethylene alkyl methyl ammonium chlorides in a herbicidal composition containing a herbicidally acceptable salt of N-phosphonomethylglycine, nor do they disclose that such herbicidal compositions have reduced eye irritation.
Accordingly, it would be desirable to obtain a stable, concentrated aqueous formulation of N-phosphonomethylglycine which contains a surfactant that at a low ratio of surfactant to N-phosphonomethylglycine is highly effective in enhancing the biological effect of N-phosphonomethylglycine but has lower eye irritation and aquatic toxicity than other surfactants. It has now been found that certain quaternary ammonium compounds are effective at low ratios of surfactant to N-phosphonomethylglycine in enhancing the herbicidal activity of N-phosphonomethylglycine, and that concentrated aqueous formulations of N-phosphonomethylglycine containing such quaternary ammonium compounds are clear, stable, of very low toxicity to aquatic life and virtually non-irritating to the eye.